The transmission of digital information and data between systems has become an essential part of commonly used systems. With such systems, information content is transmitted and received in digital form as opposed to analog form. Information long associated with analog transmission techniques, for example, television, telephone, music, and other forms of audio and video, are now being transmitted and received in digital form. The digital form of the information allows signal processing techniques not practical with analog signals. In most applications, the user has no perception of the digital nature of the information being received.
Traditional modes of communication often occur in "real time." For example, a telephone conversation occurs in real time. A "live" television sports broadcast occurs in real time. Users have come to expect these and other such traditional forms of communication to be in real time. Thus, digital transmission and reception techniques and systems need to provide for the real time transmission and reception of information.
There is a problem, however, in that digital communication between devices distant from each other usually precludes the availability of identical sampling frequencies. Except for those cases where a distinct clocking hierarchy structure can be defined and a common distributed clock source employed, there will be some difference between the sample rate of one device (e.g., the transmitter) and the sample rate of the other device (e.g., the receiver).
Prior Art FIG. 1 shows a typical prior art digital information transmission and reception system 100. In system 100, a signal source 101, for example, a video camera, generates an analog input signal. The input signal is coupled to a sampler-ADC (analog to digital converter) 102, where it is sampled and encoded into a digital pulse code modulated signal. This signal is transmitted across a transmission link to a sampler 103. Sampler 103 is coupled to a DAC (digital to analog converter) reconstruction filter 104. The sampler 103 samples the pulse code modulated signal received via the transmission link. The sampling creates a digital signal, which is subsequently coupled to the DAC-reconstruction filter where it is decoded and filtered into an output signal. The output signal represents the input signal from signal source 101.
To maintain synchronization between the devices on either side of the communications link, sophisticated synchronization technology has been developed. In most instances, the synchronization technology functions adequately. Consequently, digital communications systems (e.g., digital television, digital telephony, etc.) have proliferated and become widely accepted. The synchronization performance obtainable with conventional, prior art synchronization technology is sufficient to allow most applications (e.g., digital television) to function as intended.
Prior Art FIG. 2 shows a digital communications system 200 employing a typical prior art synchronization scheme. System 200 includes a transmitting device 201 sending a data signal to a receiving device 202. Transmitting device 201 provides a transmitter clock signal to a phase comparison circuit, phase locked loop (PLL) 203. PLL 203 generates a voltage output, V.sub.out, which is coupled to a VCO (voltage controlled oscillator) 205. V.sub.out controls the frequency of a clock signal, CLOCK A, generated by VCO 205. CLOCK A is coupled to a frequency divider 204, where it is divided, typically by some large integer factor, to produce a clock signal CLOCK B. PLL 203 compares the phase of CLOCK B and the transmitter clock and adjusts V.sub.out until CLOCK B and the transmitter clock are in phase.
When the transmitter clock and CLOCK B are in phase, PLL 203 supplies a lock indication signal to receiving device 202, informing the device it can now reliably use CLOCK B to sample the DATA signal from transmitting device 201. Only after this time (e.g., phase lock) can reliable communication occur.
It should be noted that the transmitting device 201 and receiving device 202, as with most digital communications systems, are able to adjust their clock frequencies within a certain range "F.sub.w " about a nominal frequency "F.sub.o ", at a certain rate. When communication is initiated between the transmitter device 201 and the receiving device 202, the initial phase difference between the transmitter clock and the receiver clock can be any value within a range of zero degrees to 180 degrees. Hence, based upon the rate at which the frequencies and phases can be adjusted, and based upon the size of the range, system 200 will require a significant amount of time to acquire phase lock.
For example, in case where system 200 is a DECT (Digital Enhanced Cordless Telephony) system connected to an ISDN central office branch exchange where the transmitter clock frequency=8 kHz and (F.sub.w /F.sub.o)=10.sup.-5, phase lock time may run up to seven seconds. Phase lock time can increase even more significantly if the transmitter clock frequency or the receiver clock frequency (e.g., CLOCK B) deviates from F.sub.o. Acquiring phase lock requires that the CLOCK B signal be tuned to deviate as much as possible from the transmitter clock frequency so that the phases of both frequencies approach each other as fast as possible, with CLOCK B being slowly slewed by PLL 203 and VCO 205. This resembles two trucks on an uphill highway trying to catch up with each other, having the same engine horsepower.
Phase lock has to be achieved each time the phone rings before useful communication can start. Prior to synchronization, no reliable communication can be established between the two digital telephone devices. Moreover, in some digital telephone devices, the device's specifications may even require that its communications circuits be disabled during synchronization acquisition (e.g., before a stable lock condition is achieved) because frequencies may be out of their specified range during that time.
This presents an increasingly problematic situation, in that the majority of the more modern communications devices rely upon connections that are frequently established and released and tuning ranges F.sub.w are reduced. The communications link is established as needed, as rapidly as possible, and subsequently released as rapidly as possible when no longer needed (e.g., in order best to conserve frequency bandwidth, to achieve high system utilization rates, to serve more customers, and the like).
Thus, what is required is a system for digital transmission which overcomes the slow synchronization limitations of the prior art. The required system should provide for digital transmission and reception systems which achieve rapid phase lock. The required system should be capable of rapidly establishing a stable communications link as needed. The present invention provides a novel solution to these requirements.